1. Field of the Invention
This invention relates to PTC conductive polymer devices.
2. Introduction to the Invention
Many electrical devices comprise two laminar electrodes and, sandwiched between them, a laminar resistive element which exhibits PTC (positive temperature coefficient) behavior, in particular circuit protection devices. The PTC resistive element may be composed of conductive polymer (this term being used to denote a composition comprising a polymer and, dispersed or otherwise distributed therein, a particulate conductive filler) or a ceramic, e.g. a doped barium titanate. When a conductive polymer is used, such devices are generally prepared by stamping (or otherwise cutting) a plurality of the devices out of a laminate of a sheet of the conductive polymer between two metal foils. When a ceramic is used, such devices are usually prepared by applying liquid electrode material to the major surfaces of a preformed laminar resistive element, and solidifying the liquid electrode material.
Devices of this kind are sometimes installed between two separate resilient retaining members which bear on opposite faces of the device. Typically, there are two metal retaining members which are directly opposite to each other and which bear on the outer faces of the electrodes. More specialized installations are described for example in U.S. Pat. Nos. 4,924,204 (Uchida) and 5,451,921 (Crawford et al.) the disclosures of which are incorporated herein by reference. Care is needed to ensure that an appropriate pressure is generated and maintained between the retaining members and the PTC device.
One disadvantage of some of the known installations is that the retaining members are secured to a larger structure, e.g. a motor housing, in a way that is difficult to accomplish and/or permits the retaining members to move relative to each other, thus changing the pressure that they exert on the PTC device. In other installations, the retaining members extend from the PTC element in the form of terminals (which may be male or female) and these terminals are connected to complementary connectors which are fixed to a circuit board or other rigid substrate.